fix(api): retry commands that clash with a concurrent write

Services run commands through a CommandRunner that runs one again (up to
5 times, waiting a little longer each time) when its aggregate changed
underneath, e.g. a saga writing to the same ticket. Each command loads its
aggregate fresh and stores its events in one transaction, so a
ConcurrencyError means nothing was stored and running it again is safe.
People no longer get a 409 for it, and the e2e suite stops flaking.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

libs/api/identity/application/src/lib/identity.service.ts

@@ -1,5 +1,6 @@
+import { CommandRunner } from '@task-stream/api/shared/application';
 import { Injectable } from '@nestjs/common';
-import { CommandBus, QueryBus } from '@nestjs/cqrs';
+import { QueryBus } from '@nestjs/cqrs';
 import {
   AuthorizationConfig,
   TokenGrant,
@@ -20,7 +21,7 @@ import {
 @Injectable()
 export class IdentityService {
   constructor(
-    private readonly commandBus: CommandBus,
+    private readonly commands: CommandRunner,
     private readonly queryBus: QueryBus,
   ) {}
 
@@ -44,14 +45,12 @@ export class IdentityService {
   }
 
   handleWebhook(event: string, data: Record<string, unknown>): Promise<void> {
-    return this.commandBus.execute(
-      new HandleIdentityWebhookCommand(event, data),
-    );
+    return this.commands.execute(new HandleIdentityWebhookCommand(event, data));
   }
 
   /** Sign in / refresh through the identity server (the browser cannot call it: no CORS). */
   issueToken(grant: TokenGrant): Promise<TokenSet> {
-    return this.commandBus.execute(new IssueTokenCommand(grant));
+    return this.commands.execute(new IssueTokenCommand(grant));
   }
 
   authorizationConfig(): Promise<AuthorizationConfig | null> {